Optical wavelength division multiplexed multiplexer/demultiplexer for an optical printed circuit board and a method of manufacturing the same

ABSTRACT

The invention provides an optical mux/demux for an optical printed circuit board. The mux/demux comprises: a first waveguide formed on a support layer for carrying a wavelength division multiplexed optical signal; a separator/combiner for separating the wavelength division multiplexed signal into component signals of corresponding wavelengths or for combining component signals into the said wavelength division multiplexed signal; and plural second waveguides, each for receiving or providing one or more of the said component signals, wherein the separator/combiner is at a predetermined location relative to the waveguides.

This application claims the benefit of priority of U.S. provisionalapplication Ser. No. 60/847,403, filed Sep. 27, 2006, the entirecontents of which are incorporated by reference herein.

The present invention relates to an optical wavelength divisionmultiplexed multiplexer/demultiplexer for an optical printed circuitboard. The invention also relates to an optical printed circuit boardand a method of manufacturing an optical multiplexer/demultiplexer andan optical printed circuit board.

In embodiments the invention relates to an optical wavelength divisionmultiplexed multiplexer/demultiplexer including a multimode waveguide.

The application of optical printed circuit board technology to veryshort reach (VSR) applications is a developing concept but as data ratesincrease and approach 10 Gb/s and beyond, it is likely to become anattractive option. Advantages arise with the use of opticalcommunications on a printed circuit board due to the fact that withoptical communications, unlike with electrical communications, problemsassociated with high frequency data signals such as cross-talk, EMI(electro-magnetic interference), skin effect are reduced or eliminated.In addition, the use of optical communications on backplanes enablesignificant increases in bandwidth to be achieved by permitting multiplesignals to be conveyed along common paths or waveguides.

In VSR applications the emphasis has traditionally been on low-costwaveguide fabrication processes. In our co-pending U.S. patentapplication Ser. No. 11/125,341 having the filing date 10 May 2005,there is disclosed a Coarse Wavelength Division Multiplexed system foran optical backplane. The entire content of that application is herebyincorporated by reference.

Throughout this specification the terms “multiplexer” or “demultiplexer”are used to refer to the combination of the component that performs theactual combination or separation of component signals and the waveguidesthat serve to provide inputs or receive outputs from that component.Usually, but not always, a support is provided on which themultiplexer/demultiplexer may be provided or formed.

According to a first aspect of the present invention, there is providedan optical multiplexer/demultiplexer for an optical printed circuitboard, the multiplexer/demultiplexer comprising a first waveguide formedon a support layer for carrying a wavelength division multiplexedoptical signal; a separator/combiner for separating the wavelengthdivision multiplexed signal into component signals of correspondingwavelengths or for combining component signals into the said wavelengthdivision multiplexed signal; plural second waveguides, each forreceiving or providing one or more of the said component signals,wherein the separator/combiner is at a predetermined location relativeto the waveguides, the input/output interfaces of the waveguides beingshaped to at least partially collimate light passing therethrough.

The invention provides an optical multiplexer/demultiplexer in which theinput/output interfaces of the waveguides are shaped to at leastpartially collimate light passing therethrough. Therefore, it ispossible that no other optical component may be required for use in theoptical multiplexer/demultiplexer since any required collimation of thelight may be provided by the waveguides themselves. Furthermore, as willbe explained below such an arrangement may be formed in a simple androbust manner using techniques such as lithographic formation from atwo-dimensional pattern of a mask. This enables the shape of thecollimating structure to be selected accordingly.

According to a second aspect of the present invention, there is providedan optical multiplexer/demultiplexer for an optical printed circuitboard, the multiplexer/demultiplexer comprising a first waveguide forcarrying a wavelength division multiplexed optical signal; aseparator/combiner for separating the wavelength division multiplexedsignal into component signals of corresponding wavelengths or forcombining component signals into the said wavelength divisionmultiplexed signal; plural second waveguides, each for receiving orproviding one or more of the said component signals, wherein theseparator/combiner is at a predetermined location relative to thewaveguides.

Preferably, one or all of the waveguides is or are formed on a supportlayer. Preferably, the separator/combiner is also formed on a supportlayer.

Preferably, the relative location of the separator/combiner with respectto the waveguides is determined during forming of the waveguides.

Preferably the waveguides are multimode waveguides. Accordingly, in anembodiment the invention provides a wavelength divisionmultiplex/demultiplex structure for multimode waveguides which may beimplemented lithographically on an optical printed circuit board. Theuse of multimode waveguides provides benefits in terms of the cost offabrication and the ease of optical interconnection.

In the case where a multimode waveguide is used as the input (or output)waveguide, a number of other criteria must be satisfied if optimalperformance is to be achieved. A first criterion relates to theseparation of the output interface of the waveguide and the inputinterface of the separator/combiner. A second criterion relates to thephase difference of light that exits from different positions on theoutput interface. This will be explained in greater detail below.

As explained above, in one example, the multiplexer/demultiplexer may beformed on a support. Once, formed, the support such as an FR4 PCBsupport, could be peeled or dissolved off and replaced by, for example,a suitable polymer potting compound.

Conventionally, in the case of demultiplexing when the input waveguideis a multimode waveguide it has been considered almost impossible toseparate optical signals at different wavelengths. As will be explainedbelow, this is due to the multimode waveguide allowing the light totravel at a wide range of angles within the waveguides' numericalaperture. Each wavelength travels through the waveguide and emergestravelling within the same range of angles. A second problem is that thewaveguide has a width, D, and that any one wavelength will emerge fromeach point across the output aperture of the waveguide with the fullrange of angles. So spatial position, angle and wavelength are mixed anddifficult to separate. By ensuring that the distance between the outputinterface of the input multimode waveguide and the input interface ofthe separator/combiner is above some threshold, this problem may besolved.

In an embodiment, the invention provides an opticalmultiplexer/demultiplexer in which the relative location of theseparator/combiner is defined during the forming of the waveguides.Accordingly, accurate alignment of the separator/combiner with respectto the waveguides is enabled. A simple and robust method is provided bywhich the position of the separator/combiner can be accuratelyguaranteed without requiring any complex alignment procedures.

According to a third aspect of the present invention there is provided amethod of forming an optical multiplexer/demultiplexer for an opticalprinted circuit board, the method comprising on a support layer, forminga first waveguide for carrying a wavelength division multiplexed opticalsignal; on the support layer, forming plural second waveguides forcarrying components of a said wavelength division multiplexed signal;during said step of forming the first and/or second waveguidesdetermining the relative location of a separator/combiner forseparating/combining the optical signals wherein a common mask is usedto determine the relative location of the waveguides and theseparator/combiner.

A method is provided by which an optical multiplexer/demultiplexer maybe formed in a convenient and quick manner. In particular, an opticalmask is used that includes the pattern of both the waveguides and thatof the separator/combiner or some alignment projection against which aseparator/combiner may be positioned. Thus, extremely accurate relativealignment between the separator/combiner and the optical waveguides canbe achieved.

The invention and embodiments provide a method of forming an opticalprinted circuit board in which the relative location of aseparator/combiner for a multiplexer/demultiplexer is determined duringthe formation of the optical waveguides. Thus, accurate relativealignment is assured between the optical waveguides and theseparator/combiner.

The use of a relatively simple method of manufacture enables opticalprinted circuit boards with multiplexers/demultiplexers to bemanufactured cost effectively. Furthermore, where multimode waveguidesare used, low-cost fabrication can be utilised and greater resilience toenvironmental factors is provided. In addition, the resulting structuremay be formed in a single step and therefore there is no additional costto the fabrication process to form the optical waveguides themselves. Aswell as lithographic techniques, other known techniques for forming suchoptical components may be used.

According to a further aspect of the present invention, there isprovided an optical printed circuit board comprising: a first waveguide(e.g. a multimode waveguide) for carrying a multiplexed optical signalformed of plural component optical signals; plural second opticalwaveguides (e.g. multimode waveguides) each for carrying at least one ofthe components of the multiplexed optical signal; and aseparator/combiner for separating the multiplexed signal into itscomponents or for combining the components into the multiplexed signal.

Preferably, the position of the separator/combiner is predetermined withrespect to the position of the waveguides.

According to a further aspect of the present invention, there isprovided an optical multiplexer for an optical printed circuit board,the multiplexer comprising: a first multimode waveguide for carrying amultiplexed optical signal; one or more second waveguides, each forcarrying a component of the multiplexed optical signal; and an opticalseparator/combiner for separating the multiplexed optical signal intocomponents or for combining the components into the multiplexed opticalsignal, wherein the separation between the first multimode opticalwaveguide and the optical separator/combiner is sufficient such that theangular range of direction of propagation of light entering theseparator/combiner from the first multimode optical waveguide issufficiently small to enable separation of wavelength components. Inother words, light entering the separator/combiner from the firstmultimode optical waveguide is sufficiently unidirectional forseparation.

Preferably, the distance (z) between the first multimode opticalwaveguide and the optical separator/combiner is determined by theequation:

$z > \frac{2{D^{2}\left( {\cos \; \theta} \right)}^{2}}{\lambda}$

in which

D is the diameter or width of the waveguide;

λ is the wavelength of the light in question; and,

θ is the angle that the rays in question travel from the mainlongitudinal waveguide axis.

If the input waveguide is multimode, then in order for the demultiplexerto work, the separator/combiner must be sufficiently far away from thewaveguide so that the angular range of directions of propagation of therays is sufficiently small to allow different wavelengths to beseparated in angle. If there is too much divergence of each of theindividual wavelengths in angle then the different wavelengths willoverlap and will not be completely separated.

In the case of diffractive separators/combiners (such as curvedgratings) the path lengths from each point on the output face of theinput waveguide to each point on the input face of the output waveguidemust not differ by more than a fraction of a wavelength (e.g. a smallfraction such as, say, about 0.0016×wavelength or (1/600)×wavelength=˜0.00136 μm=0.01 radians in phase=0.573° in phase) inorder for there to be the necessary constructive and destructiveinterference. This may be referred to as a “phase condition” and itapplies for both single and multimode waveguides and is a requirement ofdiffraction. This is part of the reason that the curved grating isarranged to lie along a wave front.

In one example, the maximum difference in distance from each horizontaledge of the input waveguide exit face to the nearest and furthest partof the nearest surface of the wavelength separating element to thewaveguide should be less than a fraction of a wavelength.

In the case of an arrayed waveguide grating multimode interferencecoupler region, this phase condition must be met. In the case of anarray of microprisms all of the light arriving at the output waveguidesfor any one wavelength is preferably in phase. This means that the paththrough one microprism compared to the path through the next microprismmust either be the same length or must differ by an integer number ofwavelengths.

The cut off section of the refractive Fresnel lens is also subject tothis condition as it is effectively an array of microprisms in crosssection each having a different apex angle. The cut off section ofdiffractive Fresnel zone plate is also subject to the same condition.The phase condition need not be met for the single large prism forcontinuous wave unmodulated light. The part of a lens WDM demultiplexerautomatically ensures that the phase condition is met.

A further requirement is needed if very high bit rate modulated light isused, as is anticipated, to send data. In this case a path lengthcondition must be met by all separator/combiners. The path lengths fromeach point on the output face of the input waveguide to each point onthe input face of any of the output waveguides for each wavelength mustnot differ by more than a fraction of a bit length (e.g. a smallfraction such as, say, 1/1000 of the bit length which for 10 Gb/s datais 19 μm). This requirement is less tight than the phase condition. Ifthis requirement is not met then adjacent bits in time begin to overlapin what is known as intersymbol interference (ISI) which can lead toerrors when trying to recognise the value of the bit.

The use of waveguides that are multimode enables lower manufacturingtolerances to be used and therefore reduces the cost of manufacture.

An optical printed circuit board is also provided having an optical WDMmultiplexer/demultiplexer according to any other of the aspects of thepresent invention. Other components typically provided as part of anoptical printed circuit board may also be provided.

According to a further aspect of the present invention, there isprovided an optical multiplexer/demultiplexer for an optical printedcircuit board, the multiplexer/demultiplexer comprising a firstwaveguide formed on a support layer for carrying a wavelength divisionmultiplexed optical signal; a separator/combiner for separating thewavelength division multiplexed signal into component signals ofcorresponding wavelengths or for combining component signals into thesaid wavelength division multiplexed signal; plural second waveguides,each for receiving or providing one or more of the said componentsignals, wherein the separator/combiner is at a predetermined locationrelative to the waveguides, wherein the separator/combiner is a prism ofcontinuous uniform composition throughout.

Thus, in contrast to prisms such as photonic crystal prisms, the prismused in this respect of the present invention is of simple, easy tomanufacture and cheap form.

Examples of embodiments of the present invention will now be describedin detail with reference to the accompanying drawings, in which:

FIG. 1A shows an example of a multiplexer/demultiplexer in an opticalprinted circuit board;

FIG. 1B shows a section along the line XX′ in FIG. 1A;

FIG. 2 shows an example of a micro-prism array for use in an opticalmultiplexer/demultiplexer;

FIG. 3 shows an example of a lensed grating for use as amultiplexer/demultiplexer in an optical printed circuit board;

FIG. 4 shows an example of a separator formed of a lens made of adispersive material;

FIG. 5 shows an example of a multiplexer/demultiplexer utilising acurved reflecting blazed grating;

FIG. 6 shows an example of part of an arrayed waveguide grating; and

FIG. 7 shows an example of a multiplexer/demultiplexer in which theinputs and outputs of the waveguides are tapered;

FIG. 8 is a schematic representation of an optical mask;

FIG. 9 is a schematic representation of a further example of an opticalmask;

FIG. 10 is a schematic representation of an optical component of anoptical multiplexer/demultiplexer on an optical PCB; and

FIG. 11 is a schematic representation of an optical PCB with a maskarranged over it in close proximity for the formation of a claddinglayer.

FIG. 1A shows a plan view of an optical multiplexer/demultiplexer on anoptical printed circuit board. FIG. 1B shows a section along the lineXX′ in FIG. 1A. The multiplexer/demultiplexer comprises a prism 2 formedof a dispersive material, i.e. a material in which the speed of light inthe material depends upon its wavelength, arranged within a region 4 ofan optical printed circuit board. In this example, the prism functionsas a separator/combiner.

A first waveguide 6 is provided on one side of the prism 2 and pluralsecond waveguides 8 ₁ to 8 ₃ are provided on an opposite side of theprism 2. In use, a multiplexed optical signal propagates along the firstwaveguide 6 until it reaches the end 10 of the waveguide. From there,the multiplexed optical signal including component signals at differentwavelengths λ₁ to λ₃, is transmitted across the region 4 to be incidentupon one side of the prism 2.

The wavelengths λ₁ to λ₃ of the components are different and thereforewhen the multiplexed signal is incident upon the prism 2, due to thedispersive nature of the prism, the individual components λ₁ to λ₃ willundergo different amounts of refraction. Thus, light travels through theprism and refracts again at the output surface and then travels again asufficient distance before the individual wavelength components can becoupled into the output waveguides.

Therefore, the multiplexed signal is split into its individualwavelength components. The second waveguides 8 ₁ to 8 ₃ are positionedat appropriate locations such as to receive a respective one of thedemultiplexed components signals. As shown in the example, the uppermostwaveguide 8 ₁ receives the signal at wavelength λ₁, the middle waveguide8 ₂ receives the optical signal at wavelength λ₂ and the bottom secondwaveguide 8 ₃ receives the optical signal at wavelength λ₃.

The end 10 of the first waveguide 6 may be shaped to form a lensstructure to thereby produce a collimating lensing effect such as tocounteract the divergence of the components of the multiplexed opticalsignal as it leaves the waveguide 6. The waveguide may be formedlithographically from a two-dimensional pattern on a mask and so thecollimating structure may be selected accordingly.

This means that the refractive index difference between the lensstructure and the surrounding air will be much larger than that of thestraight part of the waveguide 6 which is surrounded by cladding. Thus,significant optical wave-shaping may be achieved to produce acollimation effect. If the region 4 were filled with cladding then therelatively small refractive index difference between the waveguide coreand the cladding material could well be insufficient to allow a normallens shape to change the beam shape so dramatically as required.

The collimated beam then traverses the free space in the region 4 andimpinges upon the prism structure 2. As will be explained below, thewaveguides 6 and 8 ₁ to 8 ₃ and the prism structure 2 may be formed inthe same manufacturing step. This ensures accurate alignment or relativepositioning of the waveguides and the prism structure.

The component signals λ₁ to λ₃ undergo refraction through the prism andseparate spatially. A lens may be provided between the input and thewavelength separator to ensure that the input beam is sufficientlycollimated if the wavelength components are to be separated spatially.This level of refraction is made possible by the relatively largerefractive index difference between the polymer that forms the prism 2and the air or other suitable medium in the region 4 of the opticalprinted circuit board.

The spatially separated components λ₁ to λ₃ exit the prism structure andare received by the separate waveguides 8 ₁ to 8 ₃, each having asimilar lens termination to that of the first waveguide 6. If thecomponent signals are travelling in the desired directions then thecollimating lens structures at the end regions of the waveguides 8 ₁ to8 ₃ act to focus the collimated signals into the corresponding waveguidecores.

In a preferred embodiment, once the prism 2 and waveguides 6 and 8 ₁ to8 ₃ have been formed on the optical printed circuit board, and thecladding has been provided to define the region 4 surrounding thewaveguides and the prism, a protective barrier or lid of someappropriate form may be provided over the region 4 so as to protect theopen space between the prism and the waveguides from becoming filledwith dirt, dust or other such undesirable material.

FIG. 1B shows a section along the line X-X′ in FIG. 1. As can be seen,in the example shown, the region 4 surrounding the prism 2 contains onlyair and is free of the cladding material. This ensures a large relativerefractive index difference between the waveguides 6 and 8 ₁ to 8 ₃,prism 2 and the surrounding region to ensure that sufficient refractionof the component signals occurs so that the device operates as desired.Whatever fills the region 4, it is necessary that it has a suitably lowrefractive index. The cladding 14 is shown on top of the waveguides 6and 8 in the section shown in FIG. 1B. In practice, the claddingmaterial 14 will surround the long regions of the waveguides as shown inthe plan view of FIG. 1.

To manufacture a multiplexer/demultiplexer for an optical PCB as shownin FIGS. 1A and 1B, initially a lower cladding layer is provided on theentire upper surface of a support layer (such as FR4) of the opticalprinted circuit board. Next the waveguides 6 and 8 ₁ to 8 ₃ and theprism 2 are formed from a curable polymer.

To do this, an optical mask may be used. The optical mask preferablyincludes both the pattern of the waveguides and the prism. This meansthat extremely accurate relative alignment between the prism and theoptical waveguides can be achieved. Since the prism 2 and the waveguides6 and 8 are preferably formed using a common mask this means that thereis no difficult alignment required to form the prism once the waveguideshave been formed. This is explained in greater detail below withreference to FIG. 8.

In another example, instead of actually forming the prism from the samepolymer material used to form the waveguides, an alignment feature orprojection (not shown) may be formed in the same step as the formationof the waveguides. A separate prism can then be inserted manually inalignment with the alignment feature or projection. Thus, again thealignment of the prism is determined during formation of the waveguidesand therefore accurate alignment between the prism and the waveguides iseasily achieved. This is explained in greater detail below withreference to FIGS. 9 to 11.

The region 4 may typically be in the shape of a channel so that once thecladding has been cured in the desired regions, the uncured materialwithin the region 4 can easily be run off the resulting structure.

The multiplexed signal propagating along the primary waveguide 6includes components at different wavelengths. The component signals atthe egress point at the end 10 of the waveguide 6 will not be exactlycollimated. This is because all of the light in the waveguide travels ata range of angles and also due to diffraction at the output. Thediffraction effect is more important in single mode waveguides. Therange of angles in the waveguide is more important for multimodewaveguides.

Different wavelengths diffract by different amounts. If the lens at theend of the waveguide 10 were designed to collimate perfectly a medianwavelength to those conveyed along the waveguide 6 then some egresssignals would be slightly diverging and some slightly converging. Forthis reason, the lens in the region 10 of the waveguide 6 is preferablyformed so as to collimate the longest wavelength available and therebycause slight convergence of other wavelengths in the multiplexed signal.

The lenses formed in the end regions of the waveguides 8 ₁ to 8 ₃ areadapted to take into account the varied levels of convergence, such thataccurate imaging occurs for the wavelength in question in the receivingwaveguide. In other words, the degree of convergence provided by thelenses at the ends of the waveguides 6 and 8 ₁ to 8 ₃ are different, themagnitude of difference being dependent on the required power of therespective lenses.

So far, the description has been of the function of the device describedin FIGS. 1A and 1B as a demultiplexer since a multiplexed signalcomprising plural components is shown as travelling along the primarywaveguide and through the prism into the secondary waveguides. It willbe appreciated that the apparatus also can function in “reverse” suchthat the apparatus can serve to multiplex signals if each of thewaveguides 8 has a signal propagating in the opposite direction (rightto left in FIG. 1A). If this were to happen then the prism would serveto multiplex these signals into a single multiplexed signal propagatingright to left in the primary waveguide 6.

Preferably the waveguides are multimode waveguides. This ensures thatfabrication is simpler and optical interconnection with the waveguidesis relatively simple. Single mode waveguides could be used. Wheremultimode waveguides are used it is preferred that the separation of theoutput interface of the input multimode waveguide is sufficient suchthat the angular range of direction of propagation of light entering theseparator/combiner from the first multimode optical waveguide issufficiently small to enable separation of wavelength components. Wherethe waveguides are multimode, there are certain requirements regardingthe shape of the ends of the waveguides to ensure that collimation canoccur. This is explained in greater detail below.

Preferably, the distance (z) between the first multimode opticalwaveguide and the optical separator/combiner is determined by theequation:

$z > \frac{2{D^{2}\left( {\cos \; \theta} \right)}^{2}}{\lambda}$

in which

D is the diameter or width of the waveguide;

λ is the wavelength of the light in question; and,

θ is the angle that the rays in question travel from the mainlongitudinal waveguide axis. This is sufficient to ensure that theangular range of direction of propagation of light entering theseparator/combiner from the first multimode optical waveguide issufficiently small to enable separation of wavelength components.

In the example shown in FIGS. 1A and 1B, since the entrance face of theprism is flat, the arrangement is unlikely to meet the phase conditionunless

(i) incident light is changed from a curved to a plane wavefront or

(ii) the wavefront is made very small or

(iii) the entrance face of the prism is curved to match the curved shapeof the wavefront

The first case (i) may be achieved by placing a lens with one or twocurved surfaces between the exit of the input waveguide and thewavelength separating element and similarly between the wavelengthseparating element and the output waveguide. No lens can perfectlycollimate (make plane the wavefront) light from a multimode waveguide,however by making the lens diameter very large and placing it at itsfocal length away from the input waveguide exit face and ideally placingthe wavelength separating element at a similar distance of the focallength from the lens, the lens will form a far field pattern on theprism. However, in some cases the far field pattern may not besufficient to meet the phase condition so it may also be necessary toreduce the size of the prism to meet this condition.

In the second case (ii) as will be explained below with reference toFIG. 2, a small prism or “microprism” is used However, this microprismwill only meet part of the wavefront and so an array of microprisms ispreferably used to meet the whole wavefront. The individual microprismsare preferably placed along the curve of the wavefront. These will giveseveral diverging beams angled at different angles for differentwavelengths. A focussing lens is needed to convert the diverging beamsinto converging beams which focus on the output waveguides.

In the third case (iii) the face of the prism must be curved. In effectthe prism is part-cut from a lens made from a dispersive material.

FIG. 2. shows an example of an optical multiplexer/demultiplexer for useon an optical printed circuit board in which a microprism array 16 isprovided. As explained above, the microprism array comprises pluralprisms arranged to receive a multiplexed optical signal from a firstwaveguide 6. In the examples shown, like the example of FIG. 1, themultiplexed optical signal comprises component signals at threewavelengths λ₁, λ₂ and λ₃.

The microprism array is arranged at a separation z from the interface(output in this case) of the primary waveguide 6. For a relatively largedistance z, the waveguide 6 appears as a point source emitting curvedwave fronts 18. The microprism array 16 is made up of similar prismswith a common prism angle ω and base width b. The base width b and angleω are selected so that wave fronts are substantially constant along thewhole prism length. In other words, the individual prisms are orientedalong the direction of the curved wave fronts in such a way that lighttraverses them to maximum prism efficiency.

Each of the microprisms collects a small portion of the wave front androtates it by a small angle depending on the wavelength of the light.The total effect is an overall tilt of the wavefront, the tilt dependingon the wavelength. After traversing the microprism array, light stilldiverges but appears as if it is coming from three different waveguides,corresponding to the three component wavelengths λ₁, λ₂ and λ₃. A lens20 is positioned at the focal length between the microprism array andthe output waveguides to focus light into the output waveguides 8 ₁ to 8₃.

Although not shown in FIG. 2, a cladding may be provided over thewaveguides 6 and 8 ₁ to 8 ₃. In the example shown, no rod lenses arerequired since the lens 20 together with the microprism array 16 servesto direct and focus the demultiplexed component signals into thewaveguides 8 ₁ to 8 ₃. Again, as in the example shown in FIGS. 1A and1B, the waveguides and the prisms of the microprism array 16 may beformed from the same material in the same manufacturing step. In otherwords, it is preferred that a common mask is used to form all of theoptical components shown in FIG. 2 in the same manufacturing step.Therefore, accurate relative alignment is easily achieved.

The lens 20 can be formed during the same manufacturing step using thesame mask but alternatively an alignment feature can be formed and anexternal lens can be inserted after manufacture of the waveguides andthe prisms in the microprism array.

FIG. 3 shows a lensed grating for use as a multiplexer/demultiplexer.The optical component 22 integrates a collimating lens 24 with atransmissive grating 26. The distance d in this case is selected to belarge enough such that the waveguide 6 functions as a point source. Thewaveguide is placed at the object focus point of a collimating lenswhich converts the curved wave fronts to planar wave fronts. On theopposite side of optical component 22, a concave diffraction grating isformed. In this way, rays of different wavelength are angularlyseparated and focussed at different points as shown. Thus, the desireddemultiplexing effect is achieved.

In all of the examples described above, a multiplexer/demultiplexer foran optical printed circuit board is provided. In all of the examples, aprimary waveguide 6 for carrying a multiplexed optical signal and pluralwaveguides 8 ₁ to 8 ₃ for carrying components of the multiplexed signalmay be formed together with an optical component for performing theactual multiplexing/demultiplexing operation. The waveguides and theoptical component may be formed in the same step during manufacture,thereby ensuring accurate relative alignment between them all so as toensure that the multiplexer/demultiplexer functions as required.

In one example, instead of forming the optical component(separator/combiner) for performing the actualmultiplexing/demultiplexing operation simultaneously with thewaveguides, an alignment feature or projection (not shown) may be formedduring the same step as the formation of the waveguides. Then theoptical component for performing the multiplexing/demultiplexing can beinserted manually in alignment with the alignment projection andtherefore relative alignment with the waveguides 6 and 8 ₁ to 8 ₃ isassured.

To manufacture the multiplexer/demultiplexer, initially a lower claddinglayer is formed on a PCB support material such as FR4. Then, a layer ofcurable liquid polymer is formed on the lower cladding. This ispatterned to provide the waveguides and multiplexing/demultiplexingcomponent or a corresponding alignment feature or projection. Theuncured liquid polymer is then removed and optionally a further claddinglayer is provided in required regions of the resultant structure. Thus,since the relative alignment of the multiplexing/demultiplexingcomponent and the input and output waveguides is determined during thesame step of manufacture, accurate relative alignment is assured.

Referring now to FIG. 4, a prism is shown being part formed or cut froma lens made from a dispersive material. This will be described in detailbelow with reference to FIG. 5. The dispersive effect of the lensmaterial is usually considered to be undesirable as it causes chromaticaberration so that one wavelength is brought to a focus nearer the lensthan another focus for a different wavelength.

In this case, the effect is desirable. Moreover, in the dimension normalto the plane, if the shape of the element is also that cut from aspherical circular lens then the lens has the added benefit if it isplaced at a distance of twice the focal length from the input waveguideexit 28 and similarly from the output waveguides, that it images theoutput face of the input waveguide onto the input face of the outputwaveguide and so reduces loss by spreading out of the plane. A lensplaced in this imaging configuration also has the benefit that all ofthe optical path lengths are the same from the exit of the inputwaveguide to the inputs 30 to the output waveguides and so it canoperate at very high bit rates for the light modulation.

If the lens is replaced by a Fresnel non-diffractive lens made from adispersive material a wavelength separating element can again be made.However this will not operate at high bit rates since all of the pathsare different through each of the sub elements of the lens resulting inspreading of each pulse and overlapping of the pulses. The benefits oflow loss for out-of-plane diverging beams are retained.

If the lens is replaced by a Fresnel diffractive zone plate lens madefrom a non-dispersive material then it will also operate as a wavelengthseparating element. In this case though, the shorter wavelength or bluelight comes to a focus further from the lens than the longer wavelengthred light the opposite of the case with dispersion. The reason for thisis that diffraction is occurring instead of dispersion. However, againthis does not operate well at high bit rates since the paths ofdifferently angled rays will be different lengths between the input andoutput waveguides. The benefits of low loss for out-of-plane divergingbeams are retained.

FIG. 5 shows an example of a wavelength demultiplexer separatorincluding a curved reflecting blazed grating. It is based on a curvedideally spherical mirror which ordinarily would reflect all of thediverging rays from the input waveguide back into the input waveguide.However it has been modified with the inclusion of a blazed grating onthe surface which, by diffraction, causes different wavelengths to beadditionally reflected through different angles. Where one or more ofthe waveguides are multimode waveguides, the mirror must be placedbeyond the critical distance determined by the far field and phaseconditions to obtain the required separation of wavelengths which areclose to one another in wavelength. In contrast to the microprism array,the curved reflecting blazed grating works by diffraction and notdispersion of the light signals.

An alternative version of this system is to have a planar blazed gratingas the wavelength separating element and to place a lens between it andthe input waveguide with each element separated by the focal length ofthe lens. The lens should have a sufficiently large diameter and focallength to sufficiently collimate the light so that it is incident ontothe grating almost normally. If the grating is reflective by beingcoated in a mirror or metallic layer for example, then this combinationof lens and reflective grating can be used to replace the reflectivecurved grating in FIG. 5.

If the blazed grating is transmissive then a second lens is required onthe other side of the wavelength selective element to focus the lightinto one of several foci which are laterally displaced as shown in FIG.6. The focussing lens should have a sufficiently large diameter andfocal length to adequately focus the light and should be placed onefocal length from wavelength selective element and from the outputwaveguides.

In another embodiment, a part of which is shown schematically in FIG. 6,an arrayed waveguide grating (AWG) may be used. In contrast to knownarrayed waveguide gratings suitable for use for single mode waveguidessignificant adjustments must be made if the AWG is to be suitable foruse with multimode waveguides.

The initial beam spreading or multimode interference coupler region mustbe increased somewhat to have the critical length specified by the needto meet the far field and phase conditions. Similarly the finalfocussing region or multimode interference coupler must be lengthened tomeet the same criteria.

The intermediate/waveguides 38 are single mode as in a conventional AWG,however a two dimensional array of these is preferably provided. This isbecause the light emerging from the input multimode waveguide 34 spreadshorizontally in plane and vertically out of plane and so the input endsof the two dimensional array of single mode waveguides must be arrangedon an ideally spherical surface corresponding to the spread wavefront.This single mode waveguide array consists of multiple closely spacedwaveguides in both out of plane in in-plane array formation. At theirother ends (not shown) the single mode waveguide exits lie on aconverging wavefront, ideally spherical, so that the converging wavesagain recombine into different output multimode waveguides depending ontheir wavelengths.

In all the examples described above, the region between the end of thefirst (usually input) multimode waveguide and the WDM element need notbe filled with air, but it could be a vacuum or an inert gas or a solidsuch as a polymer. In one example this region is filled with claddingpolymer.

In this region the light spreads in the plane and also out of the plane.It is generally undesirable for the light to spread out of the plane sosome means is preferably provided to ensure this spreading does notoccur. Preferably, the light remains between a layer coincident with thetop of the core waveguide and a layer coincident with the bottom of thecore waveguide. This can be achieved in several ways. In one way, ametallised surface is put on these two planes only in the region betweenthe waveguide and the waveguides. Alternatively it may only be in theregion between the waveguide and the WDM element and similarly on theother side of it.

In one embodiment, the waveguide is placed as close to theseparator/combiner as possible whilst still satisfying the requirementof being sufficiently far to carry out adequate WDM separation.

In a further different embodiment the separator/combiner is shaped inthe vertical direction as a lens and placed 2 times the focal lengthaway from the input and output waveguides so that it images the inputwaveguide exit interface onto the output waveguides entrance interfaces.

In a variant of this, a lens is placed between the input waveguide andthe separator/combiner and between the separator/combiner and the outputwaveguides. The lens is placed a distance equal to its focal length fromthe input waveguide and also from the separator/combiner so that it actsto collimate and Fourier Transform the input light onto theseparator/combiner. The lens on the other side of the separator/combinerserves to focus the light back into the waveguide as in an expanded beamconnector configuration.

To achieve collimation a number of possible approaches may be used. Onepossible way involves putting a two surface lens between the exit of thewaveguide and the wavelength selective element separated by the focallength of the lens from each element and also a similar arrangement atthe output of the system.

As shown schematically in FIG. 7, another way involves tapering theoutput end of the input waveguide in width and, preferably thickness atan angle which is larger than any angle at which light rays aretravelling inside the waveguide. Then the end of the waveguide is curvedto form a lens but a sufficient distance must be allowed in the taperedsection to meet the requirement of far field and phase matchingconditions.

It is preferable that all elements must also be curved in theout-of-plane direction in order to offset out-of-plane divergence or areprovided with a reflective surface above and below these elements. Inone example, the reflective surfaces are metallic or are reflective dueto a refractive index difference giving total internal reflection.Alternatively a dielectric stack such as in a photonic bandgap crystalmay be used to provide the desired reflectivity.

FIG. 8 is a schematic representation of an optical mask 40 for use inthe formation of an optical multiplexer/demultiplexer. In this example,the mask is shown as an exposure mask in which cut-out regions areprovided to determine regions in an optical layer that will be fixedonce exposed to a curing radiation such as e.g. UV radiation. Theopposite arrangement is also possible in which regions of the mask thatare removed correspond to material that is to be removed from theoptical layer.

The mask has a cut-out region 42 for the formation of aseparator/combiner which in the present case would be a triangularprism. In addition, the mask includes regions 44 and 46 defining theposition and configuration of optical waveguides. As will beappreciated, where a curable liquid optical material such as a polymeris arranged as a layer on a substrate and then exposed through a masksuch as that shown in FIG. 8, the waveguides and separator/combiner willbe formed in the same step since they are all defined by the same mask.

The mask is positioned over the layer of liquid polymer and thenirradiated by, for example, ultraviolet radiation to cure the liquidpolymer in the areas defined by the cut-out regions of the mask. Therelative orientation of the separator combiner cut-out 42 and thewaveguide cut-outs 44 and 46 is defined by the mask and therefore it ispossible to achieve extremely accurate relative alignment between theseparator/combiner 42 and the waveguides 44 and 46. In particular, thereis no requirement to position and orientate a separator/combiner betweenthe waveguides 44 and 46 once the waveguides have been formed. Therelative orientation is defined by the position on the mask of therespective cut-outs, 42, 44 and 46.

The cut-outs 44 and 46 include end regions 48 which are shaped in such away that when light is projected through them and the waveguides formedon the multiplexer/demultiplexer, the shape of the input/outputinterfaces of the waveguides is such as to at least partially collimatelight passing therethrough. In other words, the interfaces of thewaveguide are in the shape of collimating lenses. In this example thelenses will be half rod or cylindrical lenses due to the lithographicmethod of manufacture, which allows only structures of uniform 2Dcross-section to be built. In other words, the uppermost surface of thestructures are in a single flat plane. Other methods such as laserablation whereby a cutting laser is used to etch away a controlled depthof material, by varying laser parameters such as power and exposuretime, could be used to create a 3D profile, at least on the top side ofthe rod lens to fashion, e.g. a quarter spherical lens (one quarter of asphere). In this case, the uppermost surface of the structures may becurved or otherwise shaped such that they are not in a single flatplane. Thus, the mask also enables any further optical component betweenthe waveguides and the separator/combiner to be dispensed with. Thus,the mask and the method of manufacture enabled by it provides a simpleand straightforward way by which an optical multiplexer/demultiplexercan be made.

FIG. 9 shows a schematic representation of another example of an opticalmask. In this case, the mask 50 includes cut-out regions 44 and 46again, defining the shape of waveguides which will be formed from acurable layer of a liquid optical polymer. In this example, there is nocut-out region which defines itself a separator/combiner. Rather, thereare cut-out regions 52 which serve to define projections against which aseparator/combiner can be aligned in due course. In other words,although no separator/combiner is actually defined by a cut-out regionon the mask, the relative configuration between the waveguides 44 and 46and the separator/combiner that will eventually be used is defined bythe mask. The relative position of the cut-out regions 52 is such as tofix the relative orientation of the separator/combiner when positionedagainst the projections that will be formed as a result of the cut-outregions 52.

FIG. 10 shows a schematic representation of the optical components of anoptical multiplexer/demultiplexer. In this example, a number ofprojections 54 can be seen formed by the cut-out regions 52 in the maskof FIG. 9. A triangular prism 56 is provided within the opticalalignment projections such that it can be appreciated that its relativeorientation with respect to the waveguides 58 and 60 is defined duringthe manufacture of the waveguides 58 and 60. Thus, extremely accuraterelative alignment between the waveguides and the separator/combiner isachieved.

FIG. 11 is a schematic representation of an optical mask 62 arrangedover the waveguides 58 and 60 and the separator/combiner 56. The mask issuitable for use in the formation of an optical upper cladding layer.The mask, in this example, includes a single region 64 which is arrangedin use to block out radiation so as to define a cut-out region 66 out ofan upper cladding layer that is provided over the waveguides 58 and 60and the separator/combiner 56. The cut-out region 66 enables there to bea large refractive index difference over the boundary of the waveguidesand the separator/combiner. Thus, the collimation and separating andcombining properties of the optical components may be maximised. In use,a protective barrier layer may be placed over the cut-out region 66 tostop the region becoming filled with dirt and dust during use.

While the present invention has been described with respect to specificembodiments and applications thereof, numerous alternatives,modifications, and applications, and variations will be apparent tothose skilled in the art having read the foregoing description. Theinvention is intended to embrace those alternatives, modifications, andvariations as fall within the broad scope of the appended claims.Accordingly, the scope of the invention should be limited only by theattached claims.

1. An optical multiplexer/demultiplexer for an optical printed circuitboard, the multiplexer/demultiplexer comprising: a first waveguideformed on a support layer for carrying a wavelength division multiplexedoptical signal; a separator/combiner for separating the wavelengthdivision multiplexed signal into component signals of correspondingwavelengths or for combining component signals into the said wavelengthdivision multiplexed signal; plural second waveguides, each forreceiving or providing one or more of the said component signals,wherein the separator/combiner is at a predetermined location relativeto the waveguides, the input/output interfaces of the waveguides beingshaped to at least partially collimate light passing therethrough.
 2. Anoptical multiplexer/demultiplexer according to claim 1, wherein theseparator/combiner is arranged with reference to an alignment feature ofthe multiplexer/demultiplexer.
 3. An optical multiplexer/demultiplexeraccording to claim 1, wherein the alignment feature comprises aprojection formed in a region between the first waveguide for carryingthe said multiplexed signal and the plural second waveguides.
 4. Anoptical multiplexer/demultiplexer according to claim 3, wherein thecombiner/separator comprises a dispersive prism aligned against theprojection feature.
 5. An optical multiplexer/demultiplexer according toclaim 1, wherein the combiner/separator comprises a microprism array. 6.An optical multiplexer/demultiplexer according to claim 5, wherein themicroprism array comprises plural prisms substantially identical to eachother.
 7. An optical multiplexer/demultiplexer according to claim 1,wherein the combiner/separator comprises one or more of a lensedgrating, a curved grating, a cut-off section of a dispersive lens and acut-off section of a diffractive or refractive Fresnel lens.
 8. Anoptical multiplexer/demultiplexer according to claim 1, wherein any orall of the waveguides are multimode waveguides.
 9. A mask for forming anoptical multiplexer/demultiplexer, the mask having features defining atleast: an input/output waveguide; one or more output/input waveguides;the position of an optical separator/combiner, wherein the shape of theinput/output interfaces of the waveguides is such as to at leastpartially collimate light passing therethrough.
 10. A method of formingan optical multiplexer/demultiplexer for an optical printed circuitboard, the method comprising: on a support layer forming a firstwaveguide for carrying a wavelength division multiplexed optical signal;on the support layer forming plural second waveguides for carryingcomponents of a said wavelength division multiplexed signal; during saidstep of forming the first and/or second waveguides determining therelative location of a separator/combiner for separating/combining theoptical signals wherein a common mask is used to define the waveguidesand the relative location of the separator/combiner.
 11. A methodaccording to claim 10, wherein during the step of forming the firstand/or second waveguides the separator/combiner is also formed.
 12. Amethod according to claim 10, wherein the separator/combiner is formedof the same material as that from which the first and plural secondwaveguides are formed.
 13. A method according to claim 10, wherein thestep of forming the first waveguide and plural second waveguidescomprises exposing a curable material through the optical mask.
 14. Amethod according to claim 13, wherein the separator/combiner is definedwith the same mask as the first waveguide and plural second waveguides.15. A method according to claim 10, comprising after the first andplural second waveguides have been formed, positioning theseparator/combiner with reference to the determined relative location ofthe separator/combiner.
 16. A method according to claim 10, wherein thestep of determining the relative location of the separator/combinercomprises forming at least one alignment feature defined by the maskduring the step of forming the first and/or second waveguides.
 17. Anoptical printed circuit board, comprising at least one opticalmultiplexer/demultiplexer according to claim
 1. 18. An optical printedcircuit board according to claim 17, comprising: a first waveguide forcarrying a multiplexed optical signal; and, plural second waveguides forcarrying component optical signals of the multiplexed optical signal.19. An optical printed circuit board according to claim 18, in which thewaveguides are provided with a cladding and in which theseparator/combiner is arranged within a channel in the cladding suchthat air or another medium is present between the waveguide input/outputinterfaces and the prism.
 20. An optical multiplexer for an opticalprinted circuit board, the multiplexer comprising: a first multimodewaveguide for carrying a multiplexed optical signal; one or more secondwaveguides, each for carrying a component of the multiplexed opticalsignal; and an optical separator/combiner for separating the multiplexedoptical signal into components or for combining the components into themultiplexed optical signal, wherein the separation between the firstmultimode optical waveguide and the optical separator/combiner issufficient such that the angular range of direction of propagation oflight entering the separator/combiner from the first multimode opticalwaveguide is sufficiently small to enable separation of wavelengthcomponents.
 21. An optical multiplexer according to claim 20, whereinthe distance (z) between the first multimode optical waveguide and theoptical separator/combiner is determined by the equation:$z > \frac{2{D^{2}\left( {\cos \; \theta} \right)}^{2}}{\lambda}$ inwhich D is the diameter or width of the waveguide; λ is the wavelengthof the light in question; and, θ is the angle that the rays in questiontravel from the main longitudinal waveguide axis.
 22. An opticalmultiplexer according to claim 20, wherein, the path lengths from eachpoint on the output face of the input waveguide to each point on theinput face of the output waveguide do not differ by more than a fractionof a wavelength.
 23. An optical multiplexer according to claim 22,wherein the maximum difference is less than 0.01 radians.
 24. An opticalmultiplexer according to claim 20, in which the path lengths from eachpoint on the output face of the input waveguide to each point on theinput face of any of the output waveguides for each wavelength does notdiffer by more than a fraction of a bit length.